1. Field of the Invention
The present invention relates to a resist composition and, more particularly, to a novel chemical amplification resist composition for forming fine or very fine resist patterns in a lithographic process. The present invention also relates to a process for the formation of such resist patterns. The resist composition and patterning process according to the present invention can be advantageously utilized in the production of semiconductor devices such as semiconductor integrated circuits, for example, LSIs, VLSIs, ULSIs and other devices, using a lithographic process.
2. Description of the Related Art
Recently, in the production of semiconductor integrated circuits, the degree of integration thereof has been notably increased and accordingly LSIs and VLSIs have been produced on a commercial scale. The minimum line width of the circuit patterns in these devices approaches the sub-half micron or quarter micron order. In other words, in the production of these high performance devices, it is required to provide a resist material which are able to form fine or very fine resist patterns.
Further, in the production of semiconductor devices, to increase a throughput capacity thereof, it is also required to increase a sensitivity of the resist material designed for fine patterning, thereby shortening the exposure time to the patterning radiation. To satisfy these requirements, there has been recently invented a chemical amplification resist material which, as is well-known in the field, comprises a photoacid generator, i.e., photoactive acid-generating compound, in addition to a base resin. Upon exposure of the resist material to patterning radiation, the photoacid generator can release an acid which can then catalytically act on the base resin, thereby ensuring a highly increased sensitivity of the resist material.
The conventional chemical amplification resist material or composition which is considered to be relevant to the present invention comprises, in combination, an alkali-soluble base resin, a dissolution inhibitor or dissolution inhibiting agent (hereinafter, briefly referred to as "SIA") which can make the resist composition alkali-insoluble, and a photoacid generator (hereinafter, briefly referred to as "PAG"). In this resist composition, an acid is generated or released from the photoacid generator as a result of patterning exposure in the initial step of the lithographic process, and the dissolution inhibitor is decomposed by the produced acid. As a result of the decomposition, the dissolution inhibitor loses its dissolution inhibiting function, thus the resist composition can exhibit good alkali-solubility depending upon the alkali-solubility of the base resin used.
The change of the solubility in alkali of the chemical amplification resist composition will be further described with reference to FIG. 1 which illustrates an acidic catalytic reaction in the chemical amplification resist composition. As illustrated, an alkali-soluble base resin (not shown), a dissolution inhibitor (SIA) containing a dissolution inhibiting group (SIG) in a molecule thereof and a photoacid generator (PAG) are mixedly contained in the resist composition as a coating. At this stage, the resist composition is insoluble in an alkaline solution.
Then, in the exposure step, the resist composition is exposed to a patterning radiation. As a result, an acid is generated or released from the photoacid generator. The acid thus produced can act against the dissolution inhibitor, thereby cleaving the dissolution inhibiting group from a molecule of the dissolution inhibitor. Since the dissolution inhibitor loses its inherent function of maintaining the alkali-insolubility of the resist composition, the exposed resist composition can change its solubility in an alkaline solution from insoluble to soluble. Accordingly, when it is developed with an aqueous alkaline solution, the exposed resist composition can provide positive resist patterns. Alternatively, if it is developed with an organic solvent having a low polarity, the exposed resist composition can provide negative resist patterns.
In the chemical amplification resist composition containing the above-mentioned components, it becomes possible to increase a sensitivity of the resist composition, because the acid released from the photoacid generator can act as a catalyst against other components in the composition, and can result in many reactions therebetween.
In addition to the improvement of the sensitivity, it is also necessary to improve the resolution of the chemical amplification resist composition, and such an improvement of the resolution largely relies upon the properties of the dissolution inhibitor which is also contained in the resist composition. Especially, in order to increase a contrast in the dissolution speed and thus attain an increased resolution in the resist composition, the dissolution inhibitor used should completely inhibit dissolution of the resist composition in an alkali prior to patterning exposure, however, after completion of the exposure, the dissolution inhibitor has to be decomposed so that the dissolution of the resist composition in the alkali can be accelerated in the absence of said dissolution inhibitor.
At present, no dissolution inhibitor capable of exhibiting both a high dissolution inhibiting function (before exposure) and a high dissolution accelerating function (after exposure), thereby ensuring a sufficient exposure contrast, is known. The well-known dissolution inhibitors have a drawback in that, before exposure, they cannot exhibit a sufficiently high capability of inhibiting dissolution of the resist composition in the alkali. For example, O'Brien et al. SPIE, Vol. 920, Advances in Resist Technology and Processing V (1988), pp. 42-50, teaches use of a dissolution inhibitor such as naphthalene-2-carboxylate, t-butoxycarbonyloxy naphthalene and others, however, these dissolution inhibitors cannot provide a good contrast of the dissolution speed before and after exposure because of its poor dissolution inhibiting action (before exposure). As a result, the resist material containing the described dissolution inhibitors can provide only insufficient resolution in the resulting resist patterns.
There are other approaches to improve the dissolution inhibitors used in the resist composition. For example, McKean et al., SPIE, Vol. 920, Advances in Resist Technology and Processing V (1988), pp. 60-66, teaches introduction of bisphenol or others in a matrix portion of the dissolution inhibitor compound. Further, there has been taught the introduction of a new dissolution inhibiting group into a molecule of the dissolution inhibitor, i.e., the use of the dissolution inhibiting group capable of being decomposed with an acid, thus accelerating dissolution of the resist composition in an alkali, such as tetrahydrofuran and others. However, these improved dissolution inhibitors are still insufficient to provide a satisfactory contrast in dissolution speed and, accordingly, a satisfactory resolution, namely, they suffer from the same problems as in the above-discussed well-known dissolution inhibitors.
In addition, it has been noted in the chemical amplification resist composition that its resolution can vary widely depending upon a diffusion distance of acid generated in the resist coating and a reaction distance of the caused chain reaction, i.e., the distance from an initial point at which the acidic catalytic reaction was started to an end point at which the reaction ceases. Immediately after exposure, no diffusion of acid is observed in the resist composition, however, the generated acid can gradually diffuse in the resist composition depending upon factors such as lapse of time and postexposure baking (hereinafter, briefly referred to as "PEB") for proceeding the acidic catalytic reaction, namely, the diffusion distance of the acid can gradually increase after exposure. Similarly, the reaction distance of the chain reaction can increase with the progress of the acidic catalytic reaction. The increased distance in the acid diffusion and chain reaction induces undesirable reduction in the resolution.
In order to avoid the reduction of the resolution due to acid diffusion and other factors, there has been suggested a method in which the unexposed resist is baked at an increased prebaking temperature to make a dense resist coating which is effective to inhibit diffusion of the acid therein and thus increase a solubility thereof. This method is enough to control the diffusion of acid to some extent, however, it is insufficient to shorten the distance of the chain reaction. Accordingly, the resulting effects are not satisfactory.
Another approach is to lower the PEB temperature, thereby shortening the distance of the chain reaction. However, due to the resulting reduction of the amount or extent of the chain reaction, this method can cause a remarkable reduction of the sensitivity of the resist. Namely, it can diminish the merits obtained by using the chemical amplification resists. In other words, for the improvement of the resolution of the chemical amplification resists, it is required that the distance of the chain reaction is shortened without reducing the amount of the reaction.
In addition to the above problem concerning the distance in the acid diffusion and chain reaction, there is another problem concerning the solubility of the dissolution inhibitor used in a resist solvent. If the dissolution inhibitor has a low solubility in the resist solvent, it is difficult to add a necessary amount of the dissolution inhibitor to the resist. An excess amount of the dissolution inhibitor can precipitate from the solvent after preparation of the resist, thus causing possible problems. The usable resist solvents include many organic solvents such as ethyl lactate, butyl acetate, propylene glycol monoethylether acetate and others, however, recently, there is a tendency that the selection of the suitable resist solvent from the usable solvents becomes difficult in view of environmental protection and the safety of users. In other words, the problem of the solubility of the dissolution inhibitor in the resist solvent has to be solved while using a limited number of the resist solvents.
The last remaining problem is related to the above-discussed problem concerning the controlled dissolution speed. In the chemical amplification resist composition, if it is intended to increase its functions and properties, the key point is whether the dissolution inhibitor used in the resist composition can completely render the composition, as a whole, alkali-insoluble. It is the fact that any dissolution inhibitor compound capable of freely controlling an alkali-solubility of the resist composition and having excellent functions and properties has not yet been suggested in the field of the chemical amplification resist composition. It should be noted that the prior art problems cannot be solved, even if the compound or molecule having a low polarity is used as the dissolution inhibitor, because the dissolution inhibitor used must have a function of attracting polar groups of the alkali-soluble base resin such as novolak resin and polyvinyl phenol resin, for example, acidic hydroxyl group, through a hydrogen bond thereof towards the dissolution inhibitor itself. The inductive production of a hydrogen bond will result if the molecule of the dissolution inhibitor used contains atoms including lone pair of electrons, however, contrary to this, there arises a problem that the substituent group having a high polarity can diminish the expected dissolution inhibiting function.